1. Field of the Invention
The present invention relates to optical glass composition having high refractive index and low dispersion.
2. Description of the Prior Art
Many known glass compositions of high refractive index and low dispersion contain an amount of thorium oxide to give to the glass compositions the desired properties of high refractive index and low dispersion. But, thorium is radioactive and harmful to the human body. Therefore, the use of a thorium compound as an ingredient of a glass composition must be avoided.
An example of a high refractive index and low dispersion glass composition containing no thorium oxide is disclosed in U.S. Pat. No. 3,080,240 (Japanese Patent Publication No. 2936/1961). This known glass composition is of the system of B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -ZrO.sub.2 -Ta.sub.2 O.sub.5 and its optical constants are 1.747 to 1.873 in refractive index (nd) and 40.0 to 52.8 in Abbe number (.nu.d). However, this glass composition has some drawbacks. It is unstable against devitrification because of its high liquid phase temperature and it is unsuitable for industrial scale mass production. Furthermore, its refractive index (nd), the upper limit of which is 1.873, is not satisfactorily high.